This invention relates to semiconductor heterostructures comprising alternating sets of layers of Silicon and Germanium (or SiGe alloys), at least one set of layers being sufficiently thin as to be in a state of elastic strain, so as to define a coherent short period superlattice. Particularly, but not exclusively, this invention relates to quasi direct band gap heterostructure devices of this kind suitable for use as light emitting devices.
Superlattices, that is, heterostructure devices comprising alternating layers of materials with differing band gaps sharing a common lattice structure, are well-known and used in the art. Where the materials comprising the alternating layers have substantially different lattice parameters, one of two situations inevitably obtains; either dislocations are generated at the interface between layers, or (if, and only if, one set of layers is sufficiently thin) one or both sets of layers may exist in a permanent state of elastic strain.
In the AlGaAs system, the mismatch between alternating layers is virtually zero, but this is not the case with Silicon and Germanium where the lattice mismatch is about 4%. This would lead to the presence of a substantial number of dislocations in conventional (long-period) superlattice structures, and since dislocations act as (and generate further) re-combination sites, such superlattices are useless for a great number of applications. There has therefore been considerable interest in the possibility of ultra-thin Silicon/Germanium superlattices, and these have been theoretically discussed for over a decade. It is only very recently, however, that manufacturing techniques have permitted the deposition of such fine layers (typically comprising one to thirty monolayers of atoms) as are necessary to produce layers which will remain in elastic strain. Further, the fine structure of such superlattices have hitherto been extremely difficult to characterise - in other words, having made such a structure, it is often not possible to tell what the structure is.
Early theories also neglected the (crucial) effect of strain on the electronic band structure of the strained layers.
There have thus been several differing theoretical models of how such structures should behave - but since experimental evidence is not generally available, such theories are of little practical guidance. Furthermore, because of the nature of the models used, even a theoretical prediction of the properties of a given structure may take days of computing time and place heavy demands on computer hardware. We have now discovered criteria that make possible the realisation of a new class of heterostructure devices.
According to the invention there is provided a semiconductor device comprising a short-period superlattice of alternating layers of first and second materials of different compositions within the Si/Ge system grown epitaxially on an (100) oriented substrate, the silicon layers being M monolayers thick, the germanium layers being N monolayers thick, M being smaller than N.
Preferably, the first material is Silicon and the second material is Germanium.
Preferably, M=2(2 m+1)+x, and N=2(2 n+1)-x where x=0 or 1, m=0, 1 or 2 and n is an integer. In a specific preferred embodiment, M=2 and N=6.
Alternatively, M=3 and N=(4 n+1), where n is an integer greater than 1.
In one embodiment, the substrate is the device substrate.
Alternatively, the substrate is a buffer layer epitaxially formed on the device substrate. The substrate may consist essentially of GaAs, or of Si.sub.1-x Ge.sub.x, where 0.5&lt;x&lt;1, and preferably 0.6.ltoreq.x&lt;1-. In this latter embodiment, the ratio of M:N approximates the ratio of Silicon to Germanium in the substrate. N values of at least some successive second material layers need not be equal.
According to one aspect of the invention, devices capable of emitting light in a directional coplanar with the layers are provided.
Although both Si and Ge are indirect bandgap materials in bulk, SiGe superlattices may be sufficiently quasi-direct to be of use as device-quality light emitting structures (that is, their optical matrix elements may approach--within a few orders of magnitude--those of a direct bandgap material).
According to another aspect of the invention, there is provided a light emitting device as recited above.
Such structures have the advantage of good compatability with existing S-based VLSI technology and potentially lower materials costs than GaAs systems.